1 Field of the Invention
The present invention relates to a superabsorbent resin composition.
2 Description of Related Art
Superabsorbent resins have been widely used as a water-absorbing material in absorbent articles in the sanitary field, such as disposable diapers for babies, adults or those suffering from incontinence, and sanitary napkins. It is known that water-soluble polymers (crosslinked polymers) constituting such superabsorbent resins undergo molecular weight reduction (degradation) and deterioration with time in the presence of radical generating species, such as hydrogen peroxide or L-ascorbic acid or a salt thereof In particular, where the superabsorbent resin is used in absorbent articles such as disposable diapers and sanitary napkins, because L-ascorbic acid or a salt thereof is present in body fluids, such as urine, blood, and perspiration, it has been a serious problem that a superabsorbent resin used in such absorbent articles undergoes degradation and deterioration with time due to the radicals generated from L-ascorbic acid or a salt thereof and reduces its capacity of retaining a body fluid.
The degradation reaction of a water-soluble polymer due to such radical generating species is conspicuous after the polymer has absorbed an aqueous liquid or a body fluid, such as urine, blood or perspiration (hereinafter referred to as a water-containing condition), especially in the co-presence of transition metal ions capable of having more than one oxidation number, such as iron ions or copper ions, in the air.
Known approaches for inhibiting the above-described degradation and deterioration of superabsorbent resins include (1) a method comprising sealing the superabsorbent resin under reduced pressure, or in a nitrogen atmosphere so as to avoid contact with air (especially oxygen), (2) a method comprising using highly purified water or raw materials so as to prevent metallic ions from entering the superabsorbent resin, (3) a method comprising adding an antioxidant or a reducing agent to the superabsorbent resin, (4) a method of adding proteins or enzymes to the superabsorbent resin, and (5) a method of adding to the superabsorbent resin, metal chelating agents, such as citric acid, (poly)phosphoric acid or a salt thereof, and ethylenediaminetetraacetic acid (EDTA) or a salt thereof.
However, the methods (1) and (2) are in many cases practically impossible to apply depending on the use of the superabsorbent resin. Methods (3), (4), and (5) that rely on conventional additives achieve some effect in suppressing degradation and deterioration of superabsorbent resins but are not always sufficient in effect. In many cases, an additive must be added in a large quantity or an additive exerting a very strong action must be used before the desired effect can be manifested. Under such circumstances, the essential physical properties or functions of the superabsorbent resin tend to be seriously ruined.
According to the inventors' study, it has turned out that the use of EDTA or sodium tripolyphosphate is not so effective in stabilizing a superabsorbent resin in the presence of an aqueous solution or water containing a radical generating species, e.g., hydrogen peroxide or L-ascorbic acid or a salt thereof.
In addition to the above-mentioned performance properties of superabsorbent resins, i.e., stability over time in a water-containing state (gel stability with time), the water absorption capacity (the amount of water absorbed), the rate of water absorption, the gel strength after swelling, liquid permeability, and the like are important requirements for superabsorbent resins. However, many of these properties conflict with each other, and it is very difficult to meet all of these requirements, which has been one of the problems in developing superabsorbent resins. For example, an attempt to increase water absorption capacity is generally accompanied by reductions in gel strength after swelling and liquid permeability.
In order to solve these problems, for example, Japanese Patent Laid-Open No. 36411/87 proposes graft-treating a carboxyl- and/or carboxylate-containing superabsorbent resin with a silane coupling agent. Japanese Patent Laid-Open No. 306118/94 proposes treating a superabsorbent resin with a titanium alkoxide. Nevertheless, these methods are still insufficient for satisfying both superabsorbent performance (e.g., a water absorption capacity) and gel stability with time after swelling.
Japanese Patent Laid-Open No. 145326/95 discloses the addition of a sulfate of a polyvalent metal selected from titanium, zirconium and vanadium to a superabsorbent polymer as one method for simultaneously improving gel strength, stability, and stickiness after water absorption.
According to the inventors' study, however, the gel stability over time achieved by this method is insufficient particularly for polymers having high water absorption capacity. Besides, addition of a polyvalent metal sulfate tends to reduce the initial rate of water absorption of the superabsorbent polymer or tends to make the polymer particles before water absorption ready to agglomerate due to the hygroscopicity of the polyvalent metal sulfate.